1. Field of the Invention
The present invention relates to a fixing roller used for fixing toner on a sheet of paper and installed on an electrophotographic device such as a copying machine, a printer, or a facsimile machine, and also the present relates to a method for fabricating the fixing roller.
2. Description of the Prior Art
A fixing roller is a structural element of an electrophotographic device and is used to fix an image on a sheet of paper or the like. In the electrophotographic device, the fixing roller is positioned in front of a pressing roller in parallel and heated at a predetermined temperature. When a sheet of paper on which toner is being attached is passed through these rollers, the toner is heated and fused on the paper.
The conventional fixing roller uses indirect heating and has an electric heater positioned at the inside of a cylindrical-shaped aluminum tube. Thus the heater heats the cylindrical-shaped aluminum tube to indirectly heat and soften the toner on the paper. Furthermore, the cylindrical-shaped aluminum tube may be coated with 4-fluorinated ethylene to prevent an unintentional or faulty transfer of the softened toner to a surface of the fixing roller (i.e., a toner offset phenomenon), which leads to degradation in image quality of the printed sheet.
The conventional fixing roller uses direct heating, has been recently developed, and comprises a machined metal core on which an insulating layer, a heating resistance element, and a coating of 4-fluorinated ethylene are applied in that order (Japanese Patent Application laying-open No. 3-80279(1991)). This fixing roller is characterized by a direct conduction of heat from the heating resistance element to the toner.
However, the above conventional rollers have the following disadvantages. In the case of the indirect-heating type, the electric heater is separated from the cylindrical-shaped aluminum tube. Thus, the electrophotographic device cannot be used efficiently because too much time is taken to heat the cylindrical-shaped aluminum tube to a predetermined temperature after electric current is initially passed through the electric heater. To improve the efficiency of the electrophotographic device, the above tube can be heated all the time, but this leads to a large power consumption Another disadvantage is the high cost of manufacturing the electric heater separately from the machining and coating of the cylindrical-shaped tube.
In the case of the direct-heating type, the fixing roller can be heated up in a short time. However, as the fixing roller is prepared by applying or coating three different layers on a surface of the metallic core, the laminated layers on the tube must be kept in absolute contact with one another, and the laminate must not thermally expand when electric current is passed through the electric heater for heating the cylindrical-shaped aluminum tube.